Allow UmaHistogramEnumeration to be used with enum classes.

base/metrics/histogram_functions.h

Index: base/metrics/histogram_functions.h
diff --git a/base/metrics/histogram_functions.h b/base/metrics/histogram_functions.h
index 58fc30173c6ea17480175440202c384b96661261..41c6aa164d591e4aaa3decf970042bf0186c2a76 100644
--- a/base/metrics/histogram_functions.h
+++ b/base/metrics/histogram_functions.h
@@ -43,7 +43,12 @@ template <typename T>
 void UmaHistogramEnumeration(const std::string& name, T sample, T max) {
   static_assert(std::is_enum<T>::value,
                 "Non enum passed to UmaHistogramEnumeration");
-  return UmaHistogramExactLinear(name, static_cast<int>(sample), max);
+  DCHECK_GE(static_cast<int64_t>(max), 0);
+  DCHECK_LE(static_cast<int64_t>(max), INT_MAX);
+  DCHECK_GE(static_cast<int64_t>(sample), 0);
+  DCHECK_LE(sample, max);

[benwells, 2017/04/06 07:30:01]

How is this? Some questions:
- do you have a better way to do this? I tried doing things with static_assert
but couldn't make it actually work. 
- I am pretty sure there aren't problems with this with numbers between
INT64_MAX and UINT64_MAX, but tell me if I'm wrong.
- Is it OK to do this in an inline function?

[benwells, 2017/04/06 07:40:24]

A smart person here suggested this:
DCHECK_EQ(max, static_cast<T>(static_cast<int>(max)))

WDYT?

[dcheng, 2017/04/06 07:47:05]

Yeah, static assert won't work here.
I think this is fine. I think the number of DCHECKs could probably be reduced
though.
DCHECK_LE(static_cast<uintmax_t>(max), static_cast<uintmax_t>(INT_MAX));
DCHECK_LE(static_cast<uintmax_t>(sample), static_cast<uintmax_t>(max));

which would cover the same thing (though it's slightly more subtle)
Yeah, it's a DCHECK so it shouldn't* affect official builds.

* of course it does sometimes. But we fixed all the ones that we can, and things
like this hopefully aren't affected.

[benwells, 2017/04/06 08:16:48]

OK ... yeah, subtle :) Thanks!

+  return UmaHistogramExactLinear(name, static_cast<int>(sample),
+                                 static_cast<int>(max));
 }
 
 // For adding boolean sample to histogram.